1. Field of the Invention
The present invention relates to a developer for developing electrostatic latent images in electrophotography, electrostatic recording and electrostatic printing. More specifically, the present invention is directed to a composition and method that provides a developer which comprises toner particles and a wax to ensure the reduction and potential elimination of image offsetting by providing proper fixing or fusing during the electrophotographic process and maintaining a stable, high quality image, during extended use.
2. Description of the Background
Visible image forming methods associated with toners using electrophotographic systems have been extensively studied and are currently widely used. Typical examples of these techniques are dual-component developing methods, which use image-forming particles and often larger carrier particles, and mono-component developing methods, which use a toner comprising only magnetic or non-magnetic image-forming particles. Details of such developing methods are described in Kirk-Othmer, Encyclopedia of Chemical Technology, 4th ed., 9:261-275 (1994).
An image forming apparatus utilizing an electrophotographic method with toner is well known. In the image forming apparatus utilizing the electrophotographic method, images are generally formed onto a sheet of copy paper through the following processes.
After uniformly charging a photoconductor that serves as an image-holding body, images are exposed onto the surface of the charged photoconductor. Attenuating electrostatic charges during the exposure of light forms a latent image. Then the electrostatic latent images are visualized by developing with toner to form a toner image. The toner images are transferred onto the aforementioned medium and thereafter fixed on it by heating, pressure or solvent vapor.
In recent years, accompanying the rapid growth of computer technology, digital copiers and printers have been developed and become widely used. In these machines, mono-component developing methods have been applied more often to reduce the number of supply parts and ease of customer maintenance requirements compared with that of the dual component method.
In the mono-component systems, toner is generally required to have good fluidity and uniform chargeability in order to form a good quality visible image as described in U.S. Pat. No. 5,802,284 incorporated herein by reference. The use of silica powder additives for toner particles to impart fluidity and chargeability properties has been widely studied and is one conventionally accepted method. Many US and international patents exist and are known that include the use of silica or silicon dioxide with toner of various compositions. A subset of these patents relates to surface treatments of silica or silicon dioxide for specific purposes to somehow enhance image quality characteristics relating to electrophotography. Examples of the use of hydrophobic silica particles for toner includes JP 46-5782 A, JP 48-47345A, and JP 48-47346A.
U.S. Pat. Nos. 5,464,722, 5,447,815, 4,868,084 5,702,858, 5,561,019, 4,902,570, 4,618,556, 5,695,902, and 6,004,711 all disclose the manufacture of toners using a form of silicone oils, varnishes, silicon dioxide particles or hydrophobic silica fine powder as some with surface treatments used as additives to enhance toner images.
The most common toner image fixing system for office and personal use printers and copiers uses a heat fixing method in which a heated device contacts a toner image on the substrate under an applied pressure. Offsetting in such a system often describes the soiling or improper marking on the imaged substrate by the toner. Cold offsetting is the term usually used to describe the soiling that occurs when the temperature of the fixing device is lower than the suitable toner fixing temperature range. In this case, insufficiently melted toner adheres to the surface of the fixing device and is subsequently deposited incorrectly onto the substrate. In contrast, when the fixing device temperature is higher than the suitable fixing range, the overly melted toner can adhere incompletely to the substrate due to a loss of elasticity resulting in adherence of toner to the fixing device. The subsequent soiling of the substrate is usually termed hot offsetting. Actual offsetting is a complex phenomenon and frequently related to many factors including surface properties of the substrate and the fixing device material, chemical and physical properties of the toner, and toner particle size.
One method which has been found to reduce or eliminate offsetting includes the use of wax additives that have low softening temperatures so the resultant electrostatically transferred toner images are fixed without smearing, improper spacing between lines and/or characters or margin offset.
In a recent JP filing, JP 10-73952, a color toner formulation with a wax with a number average molecular weight (Mn) between 1500 and 7000 is claimed to provide better results regarding fusing characteristics. Normally, lower number average molecular weight (Mn) waxes have been associated with a phenomenon known as “bleeding”. In addition, this Kokai patent claims that the ratio of branched carbons to the total carbons in the wax is between 0.5 and 20.
Sakashita, U.S. Pat. No. 5,051,331, discloses a toner comprising a binding resin and a low molecular weight olefin copolymer. Sakashita teaches the low-molecular weight olefin copolymer has at least two olefin monomer repeating units and has two or more peaks of melting at temperatures between 90 C and 170 C.
Tanikawa et al. U.S. Pat. No. 5,364,722, disclose a toner comprising a binder resin and a hydrocarbon wax, and heat-fixing methods using the toner. Tanikawa et al. teach that the binder resin may be composed of homopolymers of styrene and derivatives thereof, and styrene copolymers, such as styrene-acrylate copolymer. Tanikawa et al. further teach the hydrocarbon wax provides a differential scanning calorimeter curve showing an onset temperature of heat of absorption in the range of 50 to 100 C, and at least one heat absorption peak in the range of 70-130 C.
Hagiwara et al., U.S. Pat. No. 5,389,484, disclose a toner having a binding resin having an acid component with an acid value of from 0.5 mg KOH/g to 100 mg KOH/g, a colorant, and defined aromatic amines. Hagiwara et al. teach that the acid component of the resin interacts with the amino group of the aromatic compounds to form an amide bond thereby cross-linking the polymer chains. Hagiwara et al. further teach that this can impart a rubber elasticity to the toner, so that its anti-offset properties can be improved.
Suzuki et al., U.S. Pat. No. 5,538,828, disclose a toner resin composition comprising a binder primarily composed of vinyl copolymer and an ethylene copolymer. Suzuki et al. further teach the ethylene copolymer is prepared by copolymerizing ethylene and at least one alpha- or beta-derivative of acrylic acid or an unsaturated dicarbonic acid derivative. Suzuki et. al. further teach toner-separating agents such as a low molecular weight polyester or polypropylene wax may be added.
Taguchi et al., U.S. Pat. No. 5,466,555, disclose a releasing composition for a toner comprising a low molecular weight polypropylene and at least one modified polyolefin. Taguchi et al. teach that suitable polypropylenes include polypropylene homopolymers, and copolymers of polypropylene with one or more other monomers copolymerizable therewith, for example, ethylenes and olefins. The releasing composition may be used in toner which comprises the releasing agent, colorant, and binder resin. Taguchi et al. teach suitable binder resins include styrenic and/or acrylic resins.
Sawai et al., U.S. Pat. No. 5,565,294, disclose a toner containing a colorant, a binding resin, and a polyethylene having a melt viscosity of 22000 to 26800 mPa-s at 140 C. Sawai et al. teach that when the melt viscosity of the polyethylene is less than 2200 mPa-s at 140 C, toner components are not evenly dispersed in the kneading step in the production process of toner.
Inoue et al., U.S. Pat. No. 5,658,999, disclose production of propylene waxes by polymerizing propylene with a solid catalyst formed of a transition metal compound or a reaction product between the transition metal compound and an organometallic compound, an aluminoxane and a fine particulate carrier. Inoue et al. further discloses a toner composition composed essentially of a binder resin, a colorant, and as a releasing agent, a propylene wax.
Akimoto et al. U.S. Pat. No. 5,707,772, disclose a toner comprised of a resin, a colorant, and a releasing agent. Akimoto et. al. teach the releasing agent is a low molecular weight polyolefin polymer synthesized using a metallocene catalyst. Akimoto et al. further teach the number average molecular weight of the polyolefin is from 2000 to 10000, and the ratio of weight average molecular weight to number average molecular weight (Mw/Mn) is 1.6 to 3.5.
Osterhoudt et al., U.S. Pat. No. 5,811,214, disclose a developer comprising negatively charged toner particles comprising a polymeric binder, magnetic material, and a charge control agent wherein the toner particle surface contains particles of cerium dioxide, dimethyldichlorosilane treated silica, and dimethylsiloxane treated silica. Osterhoudt et al. teach the polymeric binder may comprise styrene and an acrylate and/or methacrylate. Osterhoudt et al. further teach that useful additives include release agents such as waxes, including copolymers of ethylene and propylene having a molecular weight of 1000 to 5000 g/mole.
Eguchi et al. U.S. Pat. No. 5,928,825, disclose a toner comprising a binder resin, a colorant, and a lubricant. Eguchi et al. teach the lubricant comprises a modified polyethylene wax obtained by grafting a monomer selected from the group consisting of styrene and unsaturated carboxylic acid onto an ethylene homo- or copolymer.
Hashimoto et al., U.S. Pat. No. 5,948,584, disclose a toner comprising toner particles containing at least a binder resin, a colorant and a wax. Hashimoto et al. teach that the binder resin comprises a hybrid component comprising a vinyl polymer component and a unit of polyester component bonded to each other.
Kuwashima et al., U.S. Pat. No. 5,952,138, disclose a magnetic developer comprising a magnetic toner containing at least a binder resin, a magnetic material, and a hydrocarbon wax which is synthesized by reaction of carbon monoxide with hydrogen or by polymerizing ethylene and which has a number average molecular weight of from 600 to 1000. Kuwashima et al., teach the wax should have an acid value of less than 2.0 mg KOH/g, and that if the acid value is higher than 2.0 mg KOH/g, the wax's interfacial adhesion to the binder resin may become so large that smearing of characters results.
Urashima, et al., U.S. Pat. No. 5,955,233, disclose a toner comprising a polymer obtained by suspension polymerization in an aqueous medium of a polymerizable monomer composition, a coloring agent, and optionally, a magnetic powder in the presence of an epoxy resin and a crystalline (meth) acrylic ester type polymer. Urashima et al. further teach an offset-preventing agent may be incorporated, and that suitable offset-preventing agent may include polyolefin wax which has a weight average molecular weight in the approximate range of 1000 to 4500, preferably 2000 to 6000, such as homopolymers of polyethylene, polypropylene and polybutylene, or olefin copolymers such as ethylene-propylene copolymer.
Livengood, et. al. U.S. Pat. No. 6,331,372, disclose a toner particulates including a wax comprising an ethylene polypropylene copolymer with a non-crosslinked copolymer other than the wax also comprising an ethylene propylene copolymer. The U.S. Pat. No. 5,707,771 and related U.S. Pat. No. 5,955,234 disclose a toner for developing an electrostatic image comprised of a binder resin, a colorant and a wax where the toner has specific rheological characteristics based on elastic storage modulus at specific frequencies where the toner shows a good fixability even at a high colorant content and shows an improved fixability during fusing that occurs immediately after power is supplied to a fixing device in a cold environment. The binder resin may preferably include a low-modulus component and a high-modulus component. The waxes may preferably include both a high-melting point wax component and a low melting point wax component.
U.S. Pat. No. 5,635,325 discloses a toner for developing electrostatic images that includes at least a binder resin, a colorant and an ester wax. The ester wax is contained in 3-40 wt. parts per 100 wt. parts of the binder resin. The ester wax includes ester compounds represented by a formula of;R1—COO—R2 wherein R1 and R2 independently denote a hydrocarbon group of 15-45 carbon atoms. The ester wax contains 50-95 wt. % thereof of ester compounds having an identical number of total carbon atoms. The toner is especially characterized by low-temperature fixability, wide non-offset temperature range, good color mixing characteristics and transparency.
U.S. Pat. No. 5,741,617 describes a toner for developing electrostatic images, which comprises a binder resin, a colorant and a wax composition, characterized in that the wax composition has a molecular weight distribution as measured by GPC containing an ester wax with a weight average molecular weight (Mw) of from 350 to 4000 and a number average molecular weight of from 200 to 4000.
U.S. Pat. No. 5,747,213 details a method of forming a color toner image where the color toner contains at least a binder resin, a colorant and wax, the wax having a molecular weight distribution measured by GPC.
Finally, U.S. Pat. No. 5,840,457 summarizes many of the solid toner wax properties found to be useful for toner resin with magnetic black toner particles that are used to help control the degree of gloss, leave little residual toner, provide a high transfer efficiency, cause little abrasion of the OPC and other cartridge components, and results in less image defects due to soiling of the members pressed against the bearing member. Several wax characteristics are summarized in U.S. Pat. No. 5,840,457 and it is indicated in this patent that low-molecular weight hydrocarbon waxes, as well as polyethylene waxes and long-chain alkyl alcohol waxes are best suited to providing efficient transfer and good gloss and with the waxes that possess physical characteristics comprising a Mw/Mn=1.0-2.0 and a DSC heat absorption main peak between 60 and 120 C.
Many references exist regarding toner compositions including wax combinations to enhance fusing performance. The use of the proper combination for each specific composition is, however, unique and complex and therefore the need for proper wax agents in current specific formulations still exists.